Add artifact metadata source to Forge Maven repo

[Polaris257]

MCP snapshot 20200514 does not have a .pom like 20201028, so artifact() must be added as a metadata source.

20200514 gives a 404:

https://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp_snapshot/20200514-1.16.3/mcp_snapshot-20200514-1.16.3.pom

20201028 is valid:

https://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp_snapshot/20201028-1.16.3/mcp_snapshot-20201028-1.16.3.pom

Adding artifact() allows 20200514 to be used like so:

mappings group: "de.oceanlabs.mcp", name: "mcp_snapshot", version: "20200514-1.16.3", ext: "zip"

(ext: "zip" is required because otherwise Gradle looks for a .jar. This is equivalent to <packaging>zip</packaging> in mcp_snapshot-20201028-1.16.3.pom.)

[shedaniel]

Is it only 20200514 that suffers from this, if so, is 20200514 a problem build?

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